1. Field of the Invention
The invention is a process for the electroless deposition of metals on nonmetallic surfaces. It involves in particular the sensitization of the nonmetallic surface for electroless deposition.
2. Description of the Prior Art
Electroless deposition of metals on nonmetallic surfaces has increased in importance in recent years because of the extensive use of such electroless deposition processes in the manufacture of printed wiring boards and related articles for use in the manufacture of electronic circuits. Electroless deposition of metals is also used for decorative plating of plastics such as for automobile parts, consumer appliances, etc. Inexpensive and reliable processes for carrying out such a procedure are in great demand. It is highly desirable from an economic point of view to develop processes for electroless plating of metal on nonmetallic surfaces which are less expensive, simpler to carry out and highly reliable. For applications relating to the fabrication of electronic circuits, good control over the deposition process is often required. For example, rapid and uniform initiation of electroless deposition assures more uniform deposition of metal which is highly advantageous for printed circuit boards to be used in electronic circuit fabrication.
A difficulty in electroless plating on nonmetallic surfaces is the preparation so as to make it catalytic for the electroless deposition process. A variety of processes have been used as evidenced by the references cited below. Particularly important is the so-called one-step process in which the sensitizer (such as SnCl.sub.2) and activator (such as PdCl.sub.2) are combined into a single solution. This type of catalytic process has the advantage of producing more active surfaces for electroless plating of metal.
The entire process of catalytic activation of dielectric surfaces using the single bath sensitizer and activator has been discussed in some detail by Rantell and Holtzman ("Role of rinsing during the activation of abs plastic using mixed SnCl.sub.2 /PdCl.sub.2 catalysts," by A. Rantell and H. Holtzman, Electroplating and Metal Finishing, Vol. 27, No. 2, February 1974, pp. 15-20 cited hereafter as Rantell I.). First the dielectric surface is physically prepared for exposure to the catalytic solution either by abrasion or etching. Some surfaces may already be in a condition suitable for sensitization and need not undergo any physical or chemical surface preparation. The surface is then cleaned, usually by a water wash and exposure to hydrochloric acid. The surface is then exposed to a mixed sensitizer/activator solution called here a colloidal catalyst solution. Thereafter, the surface is rinsed in water to remove excess colloidal catalyst solution and to fix colloidal catalyst on the surface. Next, the surface is exposed to an accelerator solution. After another water rinse, the surface is exposed to the electroless plating solution and after completion of electroless plating may again be exposed to a water wash.
The accelerator solution may consist of a variety of solutions, including dilute acids and bases. Typical acids are sulfuric acid, perchloric acid (see Plating on Plastics by Muller et al., Robert Draper LTD, Teddington, 1971, Second Edition, Chapter 11, especially page 87), hydrochloric acid (see "Electron Miscroscope Investigation of Mixed Stannic Chloride/Palladium Chloride Catalysts for Plating Dielectric Substrates" by N. Feldstein et al., Journal of the Electrochemical Society., 121, 738 (1974)), and fluoboric acid (see U.S. Pat. No. 3,532,518 issued October 6, 1970 to E. D. D'Ottavio, "Mechanism of Activation of Polymer Surfaces by Mixed Stannous Chloride/Palladium Chloride Catalysts" by A. Rantell and H. Holtzman, Transactions of the Institute of Metal Finishing, IMF 51, 62 (1973) cited hereafter as Rantell II). Other accelerator solutions used are NH.sub.4 HF.sub.2 and NaOH, disclosed in the Rantell I cited above.
Rantell and Holtzman (in the Rantell I reference) have emphasized the importance of water rinsing in the catalytic activation process. They show by way of comparative experiments that the elimination of the water rinse either after the sensitization step or after the acceleration step drastically reduces the catalytic activity of the activated surface. They also show by way of the same experiments, that the elimination of both water rinses also drastically reduces the catalytic activity of the surface. They also point out in the same reference that extensive water rinsing is necessary to prevent colloid material from the sensitization bath from getting into the accelerator bath. This produces clumps of colloidal catalyst material in the acceleration bath which under usual manufacturing conditions often gets on the surface being sensitized. This produces excessive surface roughness after electroless metal deposition. Also, dragging accelerator solution into the electroless deposition bath causes breakdown of this bath due to the presence of palladium catalyst particles.
In commercial electroless deposition processes it is highly desirable to simplify the procedure. Particularly desirable is the reduction in the number of successive steps and the increase in the reliability of the process. For example, reduction of the initiation time would lead to a more uniform metal deposition which is particularly desirable in the production of metallized surfaces in the fabrication of electronic circuits.